Advances in Chemical Vapor Deposition Printed Edition of the Special Issue Published in Materials www.mdpi.com/journal/materials Dimitra Vernardou Edited by Advances Vapor in Chemical Deposition in Chemical Vapor Advances Editor Dimitra Vernardou MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin Deposition Editor Dimitra Vernardou Hellenic Mediterranean University Greece Editorial Office MDPI St. Alban-Anlage 66 4052 Basel, Switzerland This is a reprint of articles from the Special Issue published online in the open access journal Materials (ISSN 1996-1944) (available at: https://www.mdpi.com/journal/materials/special issues/ Chemical Vapor Deposition). For citation purposes, cite each article independently as indicated on the article page online and as indicated below: LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. Journal Name Year , Volume Number , Page Range. 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Contents About the Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Dimitra Vernardou Special Issue: Advances in Chemical Vapor Deposition Reprinted from: Materials 2020 , 13 , 4167, doi:10.3390/ma13184167 . . . . . . . . . . . . . . . . . . 1 Rukmini Gorthy, Susan Krumdieck and Catherine Bishop Process-Induced Nanostructures on Anatase Single Crystals via Pulsed-Pressure MOCVD Reprinted from: Materials 2020 , 13 , 1668, doi:10.3390/ma13071668 . . . . . . . . . . . . . . . . . . 5 Huzhong Zhang, Detian Li, Peter Wurz, Yongjun Cheng, Yongjun Wang, Chengxiang Wang, Jian Sun, Gang Li and Rico Georgio Fausch Residual Gas Adsorption and Desorption in the Field Emission of Titanium-Coated Carbon Nanotubes Reprinted from: Materials 2019 , 12 , 2937, doi:10.3390/ma12182937 . . . . . . . . . . . . . . . . . . 23 Xueming Xia, Alaric Taylor, Yifan Zhao, Stefan Guldin and Chris Blackman Use of a New Non-Pyrophoric Liquid Aluminum Precursor for Atomic Layer Deposition Reprinted from: Materials 2019 , 12 , 1429, doi:10.3390/ma12091429 . . . . . . . . . . . . . . . . . . 35 Xia Liu, Lianzhen Cao, Zhen Guo, Yingde Li, Weibo Gao and Lianqun Zhou A Review of Perovskite Photovoltaic Materials’ Synthesis and Applications via Chemical Vapor Deposition Method Reprinted from: Materials 2019 , 12 , 3304, doi:10.3390/ma12203304 . . . . . . . . . . . . . . . . . . 49 Charalampos Drosos and Dimitra Vernardou Advancements, Challenges and Prospects of Chemical Vapour Pressure at Atmospheric Pressure on Vanadium Dioxide Structures Reprinted from: Materials 2018 , 11 , 384, doi:10.3390/ma11030384 . . . . . . . . . . . . . . . . . . . 67 Kyriakos Mouratis, Valentin Tudose, Cosmin Romanitan, Cristina Pachiu, Oana Tutunaru, Mirela Suchea, Stelios Couris, Dimitra Vernardou and Koudoumas Emmanouel Electrochromic Performance of V 2 O 5 Thin Films Grown by Spray Pyrolysis Reprinted from: Materials 2020 , 13 , 3859, doi:10.3390/ma13173859 . . . . . . . . . . . . . . . . . . 75 v About the Editor Dimitra Vernardou received her Ph.D. in Physical Chemistry from the University of Salford in 2005. Her thesis was entitled “The Growth of Thermochromic Vanadium Dioxide Films by Chemical Vapour Deposition”. During her Ph.D., she designed, optimized and demonstrated an atmospheric pressure chemical vapour deposition (APCVD) reactor for the growth of VO2 and V2O5 coatings. Following this, she undertook postdoctoral studies in IESL-FORTH before being a research fellow in the Centre of Materials Technology and Photonics-TEI of Crete (CEMATEP) in 2006. As a research fellow, she designed and integrated an APCVD reactor for the growth of VO2 (pure or doped with tungsten), V2O5 and WO3 as coatings for thermochromic and electrochromic applications and lithium-ion batteries. Additionally, she has experience in the structural, optical and morphological analysis of coatings. Regarding the electrochemical performance of the metal oxides as electrodes, she has carried out extensive studies using cyclic voltammetry to examine the cyclability, specific capacitance, charge and time response. For the last three years, she has continuously worked with electrochemical impedance spectroscopy to investigate the lithium-ion intercalation/deintercalation mechanism at the electrode–electrolyte interface. She has published over 70 papers in peer-reviewed scientific journals with an h-index of 29 and i10-index of 59 (Source: Google Scholar). She has supervised B.Sc., M.Sc. and Ph.D. students in energy- and environmental-related areas. She has been the principal investigator in three completed research projects involving metal oxide growth and characterization for energy and environmental applications. She currently holds the position of Assistant Professor on Materials for Electrical Energy Efficiency and Storage in the Department of Electrical & Computer Engineering of Hellenic Mediterranean University. vii materials Editorial Special Issue: Advances in Chemical Vapor Deposition Dimitra Vernardou 1,2 1 Department of Electrical and Computer Engineering, School of Engineering, Hellenic Mediterranean University, 710 04 Heraklion, Greece; dvernardou@hmu.gr; Tel.: + 30-2810-379631 2 Center of Materials Technology and Photonics, School of Engineering, Hellenic Mediterranean University, 710 04 Heraklion, Greece Received: 13 September 2020; Accepted: 15 September 2020; Published: 19 September 2020 Abstract: Pursuing a scalable production methodology for materials and advancing it from the laboratory to industry is beneficial to novel daily-life applications. From this perspective, chemical vapor deposition (CVD) o ff ers a compromise between e ffi ciency, controllability, tunability and excellent run-to-run repeatability in the coverage of monolayer on substrates. Hence, CVD meets all the requirements for industrialization in basically everything including polymer coatings, metals, water-filtration systems, solar cells and so on. The Special Issue “Advances in Chemical Vapor Deposition” has been dedicated to giving an overview of the latest experimental findings and identifying the growth parameters and characteristics of perovskites, TiO 2 , Al 2 O 3 , VO 2 and V 2 O 5 with desired qualities for potentially useful devices. Keywords: CVD; electrochromism; perovskite photovoltaic materials; TiO 2 ; Al 2 O 3 ; VO 2 ; V 2 O 5 ; computational fluid dynamics In a Chemical Vapor Deposition (CVD) process, the reactants are transported to the substrate surface in the form of vapors and gases. Although there are exceptions, the vapor of the reactive compound, usually an easily volatilized liquid or in some cases a solid, would sublime directly and is generally prepared by injection of the liquid into solvent or heated evaporators [ 1 ]. The vapor is then transported to the reaction zone by a carrier gas. The unwanted gas phase nucleation (homogeneous reaction) in CVD can be eliminated through high carrier-gas flow rates, minimum temperatures and cold wall reactors [2]. Would it be possible to assemble nanostructures with confined atomic level thickness, high specific surface area and outstanding surface chemical states at large scale and low cost? CVD is compatible with in-line manufacturing processes where material properties can be controlled with great accuracy, varying growth parameters such as temperature, precursor composition and flow rate. There are various CVD technologies including pulsed-pressure metal organic CVD, atmospheric pressure CVD, atomic layer deposition, spray pyrolysis, plasma-enhanced CVD, aerosol-assisted CVD and so on. There are so many variations on CVD technology because there is no possibility of direct control of the basic processes occurring at the deposition surface. Some of the process technologies that influence the materials’ basic characteristics and, as a consequence, their potential application, are included in this Special Issue. The review article by Liu et al. [ 3 ] reported on the perovskite photovoltaic materials, with an emphasis on their development through CVD to deal with challenges such as stability, repeatability and large area fabrication methods. In this article, one can gain a clear picture of the influence of di ff erent CVD technologies and how the experimental parameters can optimize the perovskite materials for the respective devices. Pulsed-pressure metal organic CVD (PP-MOCVD) can be utilized for the development of low-cost coatings with both macro and micro-scale, three-dimensional features. Films such as TiO 2 can be uniformly deposited with control of the nanostructure dimension and the coating thickness [ 4 ]. Towards this direction, Gorthy et al. [ 5 ] highlighted the urgent need for anti-microbial coatings due Materials 2020 , 13 , 4167; doi:10.3390 / ma13184167 www.mdpi.com / journal / materials 1 Materials 2020 , 13 , 4167 to the pandemic of COVID-19 through the growth of nanostructured TiO 2 onto handles, push-plates and switches in hospitals. The morphology nanocharacteristic is believed to be the key function for photocatalytic activity with enhanced durability. CVD at atmospheric pressure (APCVD) is a thin film deposition process with typically high deposition rates. It is an attractive method because it was designed to be compatible with industrial requirements (up-scaling at low cost and high process speed) [ 6 ]. The optimization of APCVD towards the development of high yield processes can result in the excellent controllability of the materials’ stochiometry, isolating di ff erent polymorphs of VO 2 [ 7 ]. Among the various polymorphs of VO 2 , only the monoclinic VO 2 is a typical thermochromic material [ 7 ]. In particular, it is known to undergo a reversible metal-to-semiconductor transition associated with a transformation from monoclinic to tetragonal phase at a critical temperature [ 8 ]. Therefore, the utilization of a simple, low cost process with up-scalable possibilities for the development of VO 2 coatings in thermochromic windows is a priority. In the review paper of Drosos et al. [ 1 ], the progress on experimental procedures for isolating di ff erent polymorphs of VO 2 is outlined. Additionally, the importance of understanding and optimizing the behaviour of the materials supported by modelling studies is highlighted. In that way, theory meets practice, whereas cross-check procedures take place in order to establish firm materials with advanced characteristics. Atomic layer deposition (ALD) is a process based on the gas phase chemical process in a sequential manner. The majority of ALD processes occur at temperatures > 100 ◦ C with an exception of Al 2 O 3 In particular, it can be accomplished with a variety of precursors, in relatively short times and at low temperature [ 9 ]. Xia et al. [ 10 ] reported the potential to grow Al 2 O 3 at 200 ◦ C utilizing di ff erent Al precursors via ALD. A consistent 0.12 nm / cycle on glass, Si and quartz substrates was demonstrated to give complex nanostructures with conformity, uniformity and good thickness control as a protection layer in photoelectrochemical water splitting. Spray pyrolysis is a process in which a precursor solution is atomized through a generating apparatus, evaporated in a heated reactor and decomposed on the top of the substrate into particles and thin films [ 11 ]. It is proven to be very useful for the preparation and the design of functional and versatile classes of materials at low cost and easy processing. This process can result in materials with enhanced electrochemical performance for electrochromic applications combined in layered and composite forms for higher reflective property, electrochemical stability and faster electrochromic response [ 12 , 13 ]. In Mouratis et al.’s letter [ 14 ], a new approach regarding the development of V 2 O 5 electrochromic thin films at 250 ◦ C using ammonium metavanadate in water as precursor is shown. The precursor concentration can a ff ect the morphology of the oxides, resulting in a large active surface area suitable for electrochromic applications. Conflicts of Interest: The author declares no conflict of interest. References 1. Drosos, C.; Vernardou, D. Advancements, challenges and prospects of chemical vapour pressure at atmospheric pressure on vanadium dioxide structures. Materials 2018 , 11 , 384. [CrossRef] [PubMed] 2. Pulker, H.K. Coatings on Glass , 2nd ed.; Elsevier: Amsterdam, The Netherlands, 1985; p. 139. 3. Liu, X.; Cao, L.; Guo, Z.; Li, Y.; Gao, W.; Zhou, L. A review of perovskite materials’ synthesis and applications via chemical vapor deposition method. Materials 2019 , 12 , 3304. [CrossRef] [PubMed] 4. Gardecka, A.J.; Bishop, C.; Lee, D.; Corby, S.; Parkin, I.P.; Kafizas, A.; Krumdieck, S. High e ffi ciency water splitting photoanaodes composed of nano-structured anatase-rutile TiO 2 heterojunctions by pulsed-pressure MOCVD. Appl. Catal. B Environm. 2018 , 224 , 904–911. [CrossRef] 5. Gorthy, R.; Kumdieck, S.; Bishop, C. Process-induced nanostructures of anatase single crystals via pulsed-pressure MOCVD. Materials 2020 , 13 , 1668. [CrossRef] [PubMed] 6. Malarde, D.; Powell, M.J.; Quesada-Gabrera, R.; Wilson, R.L.; Carmalt, C.J.; Sankar, G.; Parkin, I.P.; Palgrave, R.G. Optimized atmospheric-pressure chemical vapor deposition thermochromic VO 2 thin films for intelligent window applications. ACS Omega 2017 , 2 , 1040–1046. [CrossRef] [PubMed] 2 Materials 2020 , 13 , 4167 7. Piccirillo, C.; Binions, R.; Parkin, I.P. Synthesis and functional properties of vanadium oxides: V 2 O 3 , VO 2 , and V 2 O 5 deposited on glass by aerosol-assisted CVD. Chem. Vap. Depos. 2007 , 13 , 145–151. [CrossRef] 8. Vernardou, D.; Louloudakis, D.; Spanakis, E.; Katsarakis, N.; Koudoumas, E. Thermochromic amorphous VO 2 coatings grown by APCVD using a single-precursor. Sol. Energ. Mater. Sol. C 2014 , 128 , 36–40. [CrossRef] 9. Groner, M.D.; Fabreguette, F.H.; Elam, J.W.; George, S.M. Low-temperature Al 2 O 3 atomic layer deposition. Chem. Mater. 2004 , 16 , 639–645. [CrossRef] 10. Xia, X.; Taylor, A.; Zhao, Y.; Guldin, S.; Blackman, C. Use of a new non-pyrophoric liquid aluminium precursor for atomic layer deposition. Materials 2019 , 12 , 1429. [CrossRef] [PubMed] 11. Jung, D.S.; Ko, Y.N.; Kang, Y.C.; Park, S.B. Recent progress in electrode materials produced by spray pyrolysis for next-generation lithium ion batteries. Adv. Powder Technol. 2014 , 25 , 18–31. [CrossRef] 12. Ramadhan, Z.R.; Yun, C.; Park, B.-I.; Yu, S.; Kang, M.H.; Kim, S.K.; Lim, D.; Choi, B.H.; Han, J.W.; Kim, Y.H. High performance electrochromic devices based on WO 3 -TiO 2 nanoparticles synthesized by flame spray pyrolysis. Opt. Mater. 2019 , 89 , 559–562. [CrossRef] 13. Mujawar, S.; Dhale, B.; Patil, P.S. Electrochromic properties of layered Nb 2 O 5 -WO 3 thin films. Mater. Today Proc. 2020 , 23 , 430–436. [CrossRef] 14. Mouratis, K.; Tudose, V.; Romanitan, C.; Pachiu, C.; Tutunaru, O.; Suchea, M.; Couris, S.; Vernardou, D.; Koudoumas, E. Electrochromic performance of V 2 O 5 thin films grown by spray pyrolysis. Materials 2020 , 13 , 3859. [CrossRef] [PubMed] © 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http: // creativecommons.org / licenses / by / 4.0 / ). 3 materials Article Process-Induced Nanostructures on Anatase Single Crystals via Pulsed-Pressure MOCVD Rukmini Gorthy, Susan Krumdieck * and Catherine Bishop Department of Mechanical Engineering, College of Engineering, University of Canterbury, 20 Kirkwood Ave, Christchurch 8041, New Zealand; rukmini.gorthy@pg.canterbury.ac.nz (R.G.); catherine.bishop@canterbury.ac.nz (C.B.) * Correspondence: susan.krumdieck@canterbury.ac.nz Received: 1 March 2020; Accepted: 1 April 2020; Published: 3 April 2020 Abstract: The recent global pandemic of COVID-19 highlights the urgent need for practical applications of anti-microbial coatings on touch-surfaces. Nanostructured TiO 2 is a promising candidate for the passive reduction of transmission when applied to handles, push-plates and switches in hospitals. Here we report control of the nanostructure dimension of the mille-feuille crystal plates in anatase columnar crystals as a function of the coating thickness. This nanoplate thickness is key to achieving the large aspect ratio of surface area to migration path length. TiO 2 solid coatings were prepared by pulsed-pressure metalorganic chemical vapor deposition (pp-MOCVD) under the same deposition temperature and mass flux, with thickness ranging from 1.3–16 μ m, by varying the number of precursor pulses. SEM and STEM were used to measure the mille-feuille plate width which is believed to be a key functional nano-dimension for photocatalytic activity. Competitive growth produces a larger columnar crystal diameter with thickness. The question is if the nano-dimension also increases with columnar crystal size. We report that the nano-dimension increases with the film thickness, ranging from 17–42 nm. The results of this study can be used to design a coating which has co-optimized thickness for durability and nano-dimension for enhanced photocatalytic properties. Keywords: anatase single crystals; process-induced nanostructures; competitive growth; pp-MOCVD 1. Introduction Titanium dioxide has been of high interest for its photocatalytic properties under UV light since the discovery of the Honda–Fujishima e ff ect in 1972 [ 1 ]. The best known application of TiO 2 is self-cleaning glass coated with Pilkington Activ TM [ 2 ]. Anatase and rutile are the most widely researched phases of TiO 2 for photocatalytic applications. The bandgap of anatase is 3.2 eV and the bandgap of rutile is 3.0 eV. Despite the wider bandgap, anatase has high photocatalytic activity (PCA) due to higher surface-adsorption rate of hydroxyl radicals [ 3 ]. Anatase also exhibits slower charge recombination rates than rutile [ 4 ]. The majority of the studies on TiO 2 photocatalysis investigate a combination of anatase and rutile [5]. TiO 2 nanoparticles have higher specific surface area than bulk titania coatings. Nanoparticles also have a shorter exciton path length from the point of generation to the surface, resulting in lower electron-hole recombination rates for films less than 15 nm [ 6 ]. Carbonaceous TiO 2 enhances the PCA in the visible spectrum [ 7 ]. Recently, many studies have reported doping TiO 2 with noble metals and other elements to extend the bandgap [8–10]. Nanostructured Materials for Coating Applications We aim to achieve a nanostructured solid material, which would have the high active surface area and low exciton migration path of nanomaterials, but without the fabrication and handling of Materials 2020 , 13 , 1668; doi:10.3390 / ma13071668 www.mdpi.com / journal / materials 5 Materials 2020 , 13 , 1668 nanoparticles. Hashimoto et al. theorized that selective nanostructuring of TiO 2 surfaces would improve the hydrophilicity of the materials thereby making them photocatalytically superior [ 11 ]. Nanostructured TiO 2 materials were reported to exhibit improved performance when used as electrodes for lithium-ion batteries compared to electrodes consisting of nanocrystalline anatase [ 12 ]. Nanostructured materials such as mesoporous titania have demonstrated improved photoanodic e ffi ciency over Degussa P25 [ 13 ]. The main challenge with making practical use of nanostructured materials is in the processing of a solid coating layer on a substrate. Figure 1 illustrates the di ff erences between nanomaterials and nanostructured materials. Nanoparticles are processed in a hydrothermal solution and are di ffi cult to attach to a substrate. Nano rods and other 2-D structures grown on substrates have not yet been demonstrated for practical coatings. The nanostructured, multiphase, thick solid coating grown by pulsed-pressure metalorganic chemical vapor deposition (pp-MOCVD) has been demonstrated to be adherent and durable [14]. Figure 1. Illustration of ( a ) TiO 2 nanoparticles; ( b ) rutile nanorods with secondary structures grown on a substrate [15]; ( c ) nanostructured TiO 2 in a solid coating adhered to a substrate [14]. Pulsed-pressure metalorganic chemical vapor deposition (pp-MOCVD) has been used to nanoengineer solid TiO 2 coatings ( ≤ 17 μ m thick) that are composites of anatase, rutile and carbon. The coatings shown in Figure 2 have a rarely-seen columnar morphology composed of single crystal anatase and polycrystalline rutile columns [ 16 ]. Anatase columns appear pyramidal at the top and the rough dendritic structures are polycrystalline rutile (Figure 2). The thick robust coatings produced on stainless steel substrates exhibited high antimicrobial activity under visible light [ 14 ]. Thinner films on fused silica substrates have formal quantum e ffi ciency 59 times higher than the commercial photocatalyst Pilkington Activ TM [ 17 ], measured with stearic acid degradation in UV light [ 18 ]. In previous work, we used ASTAR TM analysis of TEM samples of TiO 2 films grown on stainless steel to determine that the anatase columns were single crystals, even though they are made up of nano-plates, with A[220] columnar growth direction [ 14 ]. The properties of photocatalytic materials depend on the crystallography and morphology, but there has been no research reporting a study of nanoscale feature size in TiO 2 bulk or coating materials to date. 6 Materials 2020 , 13 , 1668 Figure 2. ( a ) Cross-section morphology on a fracture surface and ( b ) plan-view surface morphology of TiO 2 films prepared by pulsed-pressure metalorganic chemical vapor deposition (pp-MOCVD). In this study, coatings of TiO 2 were produced on glass and stainless-steel substrates. All crystals exhibited the characteristic anatase mille-feuille and rutile strobili nanostructures shown in Figure 2. The thickness of the plate-like mille-feuille structures was observed to depend on the coating thickness, which is controlled by the number of precursor pulses. The thickness of the mille-feuille plates in the anatase columns is of great interest because the PCA depends significantly on the nanostructure dimension and the total surface area. The competitive columnar growth of TiO 2 anatase single crystals and the observed relation to nanostructure dimensions were investigated. 2. Experimental Details 2.1. Pulsed-Pressure MOCVD Technology The pp-MOCVD technique is a one-step deposition process that was developed by Krumdieck et al. to coat large objects such as turbine blades with thermal barrier coatings [ 19 ]. The process works by direct injection, at timed intervals, of metered volumes of precursor solution via an ultrasonic atomizer into a continuously evacuated deposition chamber. The flash evaporation of atomized liquid in the evacuated reactor chamber produces a sharp pressure spick, and produces a well-mixed reactor condition enabling the coating of complex shaped objects. The objects or substrates are placed on a susceptor that is heated with a water-cooled Cu induction coil. The Titanium tetra-isopropoxide (TTIP) precursor is decomposed when it encounters the heated substrate and forms a macroscopically uniform coating. Solvent and reactant product vapors are evacuated into a liquid N 2 cold trap. At high temperatures ( > 500 ◦ C) the reactor works in the mass-transport controlled regime with high precursor-arrival rate to the substrate surface during the peak of the pressure pulse [ 20 ]. The pulsed-pressure cycle is typically 6 seconds with less than 0.5 seconds of pressure rise, and 5 seconds of pump-down [ 21 ]. The pp-MOCVD process reduces the reactor and substrate geometry e ff ects on the deposition, making it a versatile technique to coat 3D objects. 2.2. Materials and Chemicals TiO 2 coatings were deposited on 25 mm × 25 mm × 1 mm fused silica substrates (Esco Optics, Oak Ridge, NJ, USA) and a 340 stainless steel substrate (Aperam S.A., Isbergues, France) using a TTIP precursor solution. The precursor is a dilute mixture of 5 mol% of TTIP ( > 97% Sigma Aldrich, St, Louis, MO, USA) in dry, HPLC-grade toluene with no carrier gas and no additional oxidizing agents. 7 Materials 2020 , 13 , 1668 Table 1 provides a list of samples with their identities (IDs) and the respective number of pulses. All the samples were deposited at 525 ◦ C for fused silica substrates and 500 ◦ C for stainless steel substrates. The material characterizations were carried out on 6 samples prepared on fused silica substrates with the number of pulses ranging from 150 to 1000. Fused silica is snapped to provide SEM analysis of the fracture surface and measurement of the thickness. Sample G was deposited on stainless-steel 304 with 600 pulses and characterized by focused ion beam (FIB). Table 1. Sample Identifiers, Substrates and Number of Deposition Pulses. Sample ID Substrate Number of Pulses A Fused Silica 150 B Fused Silica 200 C Fused Silica 350 D Fused Silica 500 E Fused Silica 750 F Fused Silica 1000 G Stainless Steel 600 SA Stainless Steel 400 SB Stainless Steel 735 SC Stainless Steel 909 2.3. Characterization Methods The plan-view and fracture surface morphologies of the TiO 2 coatings were observed using a JEOL 7000F Scanning Electron Microscope (SEM, UC, Christchurch, NZ). The samples were scored on the uncoated side using a diamond-tipped scribe and fractured into four sections to expose the cross-section of the coatings. Prior to imaging, the fractured samples were sputtered with Cr using a Quorum Tech rotary pumped coater (UC, Christchurch, NZ). Ten measurements of the film thicknesses were obtained and the mean film thickness ( w ) reported. The mean growth-rate ( GR ) of a coating prepared with N pulses was calculated using GR = w N (1) The mean anatase column diameter was determined from the plan-view SEM images using the ASTM E112 standard circle-intercept method [ 22 ]. Five test circles were used to obtain the column size for each sample. The phase composition of the coatings was determined using a Rigaku SmartLab X-ray di ff ractometer (XRD, UC, Christchurch, NZ) equipped with a Cu K α ( λ = 1.5148 Å) source. The as-deposited samples were mounted on a flat sample holder and the detector was set up to collect from 5 ◦ to 90 ◦ in 2 θ at a scan rate of 10 ◦ per minute. The spectral peaks were indexed using the RRUFF database [23]. The chemical composition of the TiO 2 samples was analyzed using Surface Enhanced Raman Spectroscopy (SERS. The Raman spectra were obtained using a HORIBA Jobin-Yvon LabRam spectrometer (MacDiarmid Inst. / GNS, Wellington, NZ) equipped with an Ar ion (514 nm) laser). The power of the laser was set at 420 μ W and the sample surfaces were analyzed as-deposited. The spectra were deconvoluted using a Gaussian peak-fitting algorithm in Origin Pro software (OriginLab, Northampton, MA, USA). The cross section of sample G was studied using a JEOL 300CF Scanning Transmission Electron Microscope (STEM, IMRI, UC-Irvine, Irvine, CA, USA). The STEM is equipped with a 300 kV cold field emission gun and has a resolution of 80 pm. The sample for STEM imaging was prepared using a FEI Quanta 3D focused ion beam (FIB)-SEM (IMRI, UC-Irvine, Irvine, CA, USA). 8 Materials 2020 , 13 , 1668 2.4. Nanostructure Dimension Measurement The anatase columns are composed of nanoscale plates. We used a straight-forward measurement technique using the plan-view SEM images to determine the thickness of the plates. Fifteen crystals were selected from the SEM image that were symmetrical and had the most plates visible. The central plates at the peak of the crystals are always slightly thicker, so 3–5 plates as shown in Figure 3 were measured using the GMS Digital Micrograph Suite [24]. Figure 3. Methods to determine the nanoplate thickness for anatase columns. Two measurements were taken for each crystal column as shown in Figure 3. If ‘n’ plates are measured on each side of the central plates, then the thickness of a single nanoplate ‘t’ is calculated as t = 1 / 2 n ( d 1 + d 2 ) (2) The measurements are statistically analyzed to obtain the thickness of the nanoscale plates on the anatase columns and the measurement accuracy. This measurement is referred to as the nano-dimension. 3. Results 3.1. Phase and Composition of TiO 2 Coatings Prepared by pp-MOCVD The XRD analysis of the coatings showed that all the coatings were TiO 2 with both anatase and rutile phases. The lattice parameters are consistent with stoichiometric TiO 2 [ 25 , 26 ]. The XRD pattern given in Figure 4a is representative of all the measurements for the samples in this study. The pattern also shows that the anatase phase exhibits a strong [220] growth texture. This is consistent with previous studies of pp-MOCVD films [14,18,20,27]. A Raman spectrum representative of all the samples in this study is provided in Figure 4b. The deconvoluted peaks correspond to the anatase phase of TiO 2 and amorphous carbon present in the films [ 23 , 28 , 29 ]. The amorphous C peak D 1 represents aromatic carbon rings and the peak G 1 represents the sp 2 C = C bonds. No evidence for Ti-C was detected by Raman spectroscopy and no bulk titanium carbide phases were identified in XRD. These results are consistent with our previous work, where a uniform distribution of carbon was measured through the films thickness [27]. 9 Materials 2020 , 13 , 1668 Figure 4. ( a ) XRD pattern and ( b ) Raman spectrum of TiO 2 coatings prepared by pp-MOCVD. 3.2. Plan-View Surface Morphologies of TiO 2 Coatings The TiO 2 coatings were all macroscopically uniform, adherent and black in color. The SEM images of the coatings showed that the coatings were composed of columnar crystals with two distinct morphologies, anatase mille-feuille and rutile strobili as described in previous work [ 14 ]. The top of an individual anatase column resembles a plated pyramid-like structure. Figures 5 and 6 show the plan-view surface SEMs of the samples deposited on fused silica and stainless steel substrates. The figures show that the surface morphologies for TiO 2 coatings on stainless steel and fused silica substrates are similar. The images show that as the number of pulses, and thus the thickness, increases, the number of nanoscale plates and the column diameter also increase. The thickness of the plates is not obviously di ff erent, but the measurements reveal a definite trend in nanoscale dimension. The column diameters and the surface-nanostructure dimensions for samples A-F are provided in Table 2. 10 Materials 2020 , 13 , 1668 Figure 5. Plan-view SEM images showing the surface of anatase crystals for TiO 2 samples on fused silica prepared with ( A ). 150 pulses; ( B ). 200 pulses; ( C ). 350 pulses; ( D ). 500 pulses; ( E ). 750 pulses and ( F ). 1000 pulses. Figure 6. Plan-View SEM images showing the surface of anatase crystals for TiO 2 samples on stainless steel prepared with SA. 400 pulses; SB. 735 pulses and SC. 909 pulses. Table 2. Column diameters and nanoplate thicknesses reported as mean and standard deviation of the measurements. ID Number of Pulses Column Diameter (nm) Plate Thickness (nm) A 150 157 ± 35 17.85 ± 3.5 B 200 257 ± 5 27.98 ± 5.3 C 350 277 ± 5 27.53 ± 2.3 D 500 469 ± 30 36.23 ± 2.7 E 750 553 ± 10 43.25 ± 3.5 F 1000 600 ± 18 43.74 ± 3.0 11